An optical disk player employs a tilt servo device (in order to reduce cross talk) which automatically operates to control the relative position of an optical disk (hereinafter referred to merely as "a disk" when applicable) and a pickup so as to apply a data reading light beam perpendicularly to the recording surface of the disk. One example of the tilt servo device has been disclosed by Japanese Utility Model Application No. 27231/1988.
A tilt servo device comprises: a tilt sensor for providing a tilt error signal indicating the inclination of a disk: a servo amplifier for amplifying the tilt error signal; a tilt servo motor for generating a drive in a forward or reverse direction according to the output of the servo amplifier; and a tilt adjusting mechanism for performing tilt adjustment with the aid of the drive thus generated.
A related tilt servo device of this type will be described with reference to FIG. 4.
As shown in FIG. 4, a tilt sensor 1 is made up of a light emitting element LED; and two light receiving elements PD.sub.1 and PD.sub.2 arranged radially with respect to a disk 20 in such a manner they are positioned on both sides of the light emitting element LED. A light beam emitted by the light emitting element LED is reflected from the recording surface of the disk 20 and received by the light receiving elements PD.sub.1 and PD.sub.2. The quantity of light received by each of the light receiving elements changes with the tilt angle of the disk 20. The difference between the quantities of light received by the light receiving elements is input to an inverting phase input terminal of an equalizer amplifier 2, with this difference signal being referenced as the tilt error signal.
The equalizer amplifier 2 comprises: an operational amplifier OP.sub.1 ; resistors R.sub.21 and R.sub.22 connected to the inverting phase input terminal of the operational amplifier OP.sub.1, thus forming an input circuit: and resistors R.sub.23 and R.sub.24 and a capacitor C.sub.21 connected between the inverting phase input terminal and the output terminal of the operational amplifier, thus forming a feedback impedance circuit. The capacitor C.sub.21 functions electrically to disconnect the resistor R.sub.23 at low frequencies. Therefore the frequency characteristic of the equalizer amplifier 2 has two break points f.sub.2 and f.sub.3 ; for instance the break point f.sub.2 is 10 Hz, and the break point f.sub.3 is 280 Hz.
The output of the equalizer amplifier 2 is applied through a relay switch 3 to another equalizer amplifier 4. The relay switch 3 is operated under the control of a control section 5 comprising for instance a microprocessor. For instance when no disk is loaded or the pickup is located at the innermost or outermost periphery of the disk 20, the relay switch 3 is opened to eliminate the difficulty that external light or the mirror surface of the disk cause the tilt sensor to output erroneous outputs: that is, it functions to prevent the erroneous operation of the tilt sensor. The relay switch 3 is kept closed while the device is in a search operation mode (described later). When the relay switch 3 is opened, no tilt adjustment is carried out.
The equalizer amplifier 4 comprises: an operational amplifier OP.sub.2 ; a resistor R.sub.41 connected to the inverting phase input terminal of the operational amplifier OP.sub.2, thus forming an input circuit; and a parallel circuit of a resistor R.sub.42 and a capacitor C.sub.41 which is connected between the inverting phase input terminal and the output terminal of the operational amplifier OP.sub.2 thus forming a feedback impedance circuit. The equalizer amplifier 4 serves as a low-pass filter (hereinafter referred to as "an LPF", when applicable), and its output is decreased at a rate of 6 dB/Oct beginning with a frequency f.sub.1 which is for instance 0.5 Hz.
The output of the equalizer amplifier 4 is applied through a resistor R.sub.61 to a tilt drive circuit 6. The tilt drive circuit 6, comprising a pair of complementary output transistors 62 and 63, applies a forward or reverse current to a tilt servo motor 7 according to the output of the equalizer amplifier 4. The tilt servo motor 7 drives tilt adjustment mechanism (not shown) to perform tilt control. The equalizer amplifiers 2 and 4 and the tilt drive circuit 6 form a feedback circuit.
A read pickup (not shown) applies a light beam to the disk 20 and subjects the light beam reflected therefrom to electric conversion to form a read signal (hereinafter referred to as "an RF signal", when applicable). The RF signal is applied to address detecting means, namely, a data demodulating circuit 8. The circuit 8 separates control data such as for instance an address signal from the RF signal and applies it to a control section 5. A variety of instruction signals, such as a performance instruction and a search instruction, are applied to the control section 5 with a key board 9. The control section 5 provides a control output according to a control program stored in advance, in response to the control data and the instruction signal. For instance when a search instruction is produced with the key board 9, the distance between a present address supplied by the data demodulating circuit 8 and a target address specified is calculated, and a carriage drive instruction signal having a width corresponding to the distance thus calculated is applied to a carriage drive circuit 10. The carriage the carriage drive circuit 10, is applied to a carriage motor 11. As a result, a pickup carriage (not shown) adapted to position the pickup radially with respect to the disk is moved to effect the target address. Thereafter, the control section 5 operates a tracking servo (not shown) and calculates the difference in distance between the target address and the present address which is obtained from the RF signal after the tracking servo loop is locked in. The control section 5 repeatedly issues an instruction signal for a track jumping operation corresponding to the difference in distance thus calculated, and then outputs an instruction signal for a play mode when the present address approaches the target address. Thus, the search operation mode is ended. After the search operation mode, a play operation mode for the ordinary disk performance is effected. In the play operation mode, the carriage drive circuit 10 drives the carriage motor 11 when an error signal in a tracking servo system (not shown) increases in level.
The transfer frequency gain characteristic of the equalizer amplifiers 2 and 4 is shown in FIG. 5.
In the characteristic shown in FIG. 5, the frequencies f.sub.1, f.sub.2 and f.sub.3 of the three break points are 0.5 Hz, 10 Hz and 280 Hz, respectively. The gain is the highest between DC (0 Hz) and the frequency f.sub.1, and it decreases with 6 db/Oct between frequencies f.sub.1 and f.sub.2, and it decreases with 12 dB/Oct between the frequencies f.sub.2 and f.sub.3.
The reason why the total transfer characteristic of the equalizer amplifiers is of a narrow bandwidth as described above is that it is essential to minimize the tilt angle of the disk and to stabilize the tilt servo loop.
In a search operation in which, upon specification of a performance start address, search is made between the inner and outer peripheries of the disk, and the performance is started from the address thus specified, it is essential to maintain the picture quality unchanged immediately after the search operation, to suppress the erroneous counting of cross tracks, to stabilize the multi-track jumping operation, and to suppress the defocusing phenomenon in which the focusing servo cannot achieve its follow-up operation satisfactorily.
However, in the ordinary play operation, the transfer characteristic of the tilt serve system is so designed that the dead zone (steady angular deviation) is minimum in order to prevent the cross talk recognized on the picture; and for the purpose of stabilization of the tilt servo system, the bandwidth is very low and the response speed is also low. Therefore, the stability of the tilt servo at the end of the search operation and that of the tilt servo in the ordinary play operation are not compatible with each other.